The present invention relates to an acidic amorphous silica-alumina having a large specific surface area and a large pore volume, a carrier complex and a hydrotreating catalyst containing the same, in particular a hydrocracking catalyst containing the acidic amorphous silica-alumina in combination with a modified zeolite-Y for treating petroleum hydrocarbon materials to produce middle distillates, and processes for the preparation thereof.
The amorphous silica-alumina, as one-of the important support components of catalysts, is required to have an adequately large specific surface area and large pore volume to support active metal components. Additionally, the amorphous silica-alumina also needs generally to provide a certain acid cracking function for efficient reaction. However, none of the disclosed products of the prior arts possesses both functions. Commonly, the amorphous silica-alumina used as an acidic component should be dispersed in a matrix of refractory oxides having a large specific surface area and a large pore volume, such as a large pore alumina, which may serve jointly as a catalyst support. Due to limitations in the preparation processes of the prior art, when an amorphous silica-alumina which is required to have a desired pore structure that can be used directly as a catalyst support is prepared, silica introduced into it cannot be sufficient, and consequently the number of acid sites is low; on the other hand, if sufficient silica is added to increase the number of acid sites, it is liable to crystallize and will cause damage to the pore structure. Particularly, with respect to the mid-distillate type hydrocracking catalyst, in addition to the activity of the catalyst, its mid-distillate selectivity should be especially taken into consideration. Thus, in addition to a certain amount of acid sites, the carrier component is required to have comparatively good properties in respect of pore structure, including a larger specific surface area and more effective pore volume to improve the dispersion of active metal components in the carrier and to modify the diffusion properties in the coursr of reaction. The application of zeolite component in catalysts enhances the activity level of the catalysts, in particular that of the hydrocracking catalysts. The application of zeolite component in catalysts marks a great progress in the field of hydrocracking catalyst as compared with the catalysts comprising solely amorphous silica-alumina. Zeolite is first applied in light-distillate type hydrocraking catalysts, and latter in mid-distillate type hydrocraking catalysts. However, with respect to a mid-distillate type hydrocraking catalyst, the activity may not be the most important element to be considered, but the mid-distillate selectivity is very important. It is true that a large amount of zeolite can enhance the cracking activity of the catalyst, but its excessively strong cracking activity may result in noticeable decrease in the mid-distillate selectivity. Therefore, it needs to seek a better balance between the activity and the mid-distillate selectivity of the catalyst.
GB 2166971A discloses a silica-alumina cogel having a specific surface area of 100-400 m2/g, of which 30-60% is contributed by the pores having a diameter of about 4-10 nm, and a process for the preparation thereof, comprising first mixing an alkali metal aluminate and silicate to obtain a mixture having a pH of about 12 to 12.5, and then reacting the mixture with aluminum sulfate to obtain the resultant product. From the examples of the application, it can be learned that the typically resultant product SiO2xe2x80x94Al2O3xe2x80x94Re2O3 cogel has a specific surface area of 277 m2/g, a pore volume of 0.31 ml/g and an average pore diameter of 6.1 nm respectively under the conditions that its SiO2 content is less than 35 wt %. Although the said product possesses cracking activity, it has the defects of being small in pore volume and specific surface area and cannot be used as a carrier alone for supporting a large amount of metal components.
U.S. Pat. No. 4,758,330 discloses a catalyst composition having a high surface area and a high pore volume comprising alumina as the major component and silica as a minor modifying component, and also discloses a process for preparing an amorphous silica-alumina by first preparing an alumina hydrogel by pH swing preparation in a range of 2 to 10, and then adding to the hydrogel a silicate to obtain the amorphous silica-alumina. The process is complicated in operation and difficult to control, and moreover the obtained product has a low SiO2 level (less than 8.7 wt %), so it is generally used as a carrier of a hydrofining catalyst, which is not required to have the function of acidic cracking. Although this product has a pore volume of up to 1.78 ml/g, its pore volume is contributed substantially by large pores of 10-100 nm, and only 0.31 ml/g by pores of 3-10 nm, and this product has a specific surface area of 308 m2/g.
EP 0,228,270 describes a process by which alumina is treated with sodium silicate or silica sol, and is then subjected to hydrothermal treatment to improve the properties of the amorphous silica-alumina and to give the final amorphous silica-alumina having a SiO2 content of 6-12 wt %. In an embodiment of the invention, the alumina is treated with a silica sol, and then the resultant amorphous silica-alumina containing 12 wt % of SiO2 is treated by a hydrothermal method in order to give a product having the highest specific surface area, however, even in this case, the specific surface area is only 214 m2/g. The operational conditions of the process described above are rigorous and energy consumption in the hydrothermal treatment is large.
U.S. Pat. No. 4,517,074 discloses a hydrocracking catalyst for producing 300-700xc2x0 F. (149-371xc2x0 C.) middle distillates, wherein the feedstocks are preferably pretreated by hydro-denitrogenation and/or hydro-desulfurization. Said catalyst is characterized in that it comprises, in addition to a zeolite component, a dispersion of SiO2-Al2O3 in alumina matrix, wherein the amorphous silica-alumina having a certain level of cracking activity has to be dispersed into an alumina matrix, such as large-pore xcex3xe2x80x94Al2O3 used as a component of the carrier, so as to obtain sufficient pore volume and specific surface area for supporting a large amount of metal components for hydrogenation, according to its embodiment example, the activity of catalyst is relatively low, thus, in order to achieve a conversion of 60 vol %, the reaction temperature must generally be higher than 400xc2x0 C. While the reaction temperature is still needed to be kept at 389xc2x0 C. even though an example of the patent showed that by the use of a catalyst comprising a silica-alumina containing as high as 75% of silica dispersed in xcex3xe2x80x94Al2O3 in combination with a zeolite, such as zeolite LZ-10.
Therefore, there is still a need in the art to develop a hydrocracking catalyst having both good activity and high mid-distillate selectivity, and a process for the preparation thereof.
After extensive studies and experiments, the inventors have invented an acidic amorphous silica-alumina having a large specific surface area and a large pore volume and a process for the preparation thereof. When used as a carrier component, preferably in combination with a zeolite-Y, the acidic amorphous silica-alumina will provide a hydrocracking catalyst with the desirable properties including both good activity and high mid-distillate selectivity.
An object of the present invention is to provide an amorphous silica-alumina having a large specific surface area and a large pore volume and an adequate content of SiO2 to ensure a desired acid function for chemical reactions, which is particularly useful as a carrier component in the preparation of a hydrocracking catalyst having both good activity and high mid-distillate selectivity.
Another object of the present invention is to provide a carrier complex comprising an acidic amorphous silica-alumina of the present invention, which is particularly useful as carrier component in the preparation of a hydrocracking catalyst having both good activity and high mid-distillate selectivity for producing mid-distillate.
A further object of the present invention is to provide a hydrocracking catalyst, comprising hydrogenation metal components, a specific amorphous silica-alumina, and optionally a crystalline compound including a zeolite, which has both good activity and high mid-distillate selectivity and is particularly useful in treating heavy hydrocarbon materials to produce middle distillates.
Anther still further object of the present invention is to provide a hydrocracking catalyst, comprising hydrogenation metal components, a specific amorphous silica-alumina, and a modified ultrahydrophobic zeolite-Y, which has both good activity and high mid-distillate selectivity and is particularly useful in treating heavy hydrocarbon materials to produce middle distillates.
A still further object of the present invention is to provide a process for the preparation of an acidic amorphous silica-alumina having a large specific surface area and a large pore volume, with simplified procedures and lower costs for starting materials and lower energy consumption.
Still another object of the present invention is to provide a process for the preparation of the catalyst of the present invention, comprising the steps of mixing a specific amorphous Silica-alumina, optionally a crystalline compound including a zeolite, and optionally a binder, kneading, rolling, pressing and moulding into carrier, and then supporting active hydrogenation metals onto said carrier to obtain the catalyst of the invention.
These and other objects of the present invention will become apparent to the person skilled in the art after reading the specification.